1. Field Of The Invention
The present invention relates generally to methods and apparatus for perforating wells.
2. Description Of The Prior Art
One operation commonly performed in the completion of an oil or gas well is the perforation of the steel casing of the well to communicate the well bore with a subsurface formation intersected by the well. Thus, formation fluids are allowed to be produced from the formation through the perforations and up through the well bore.
Many techniques have been used in the past to actuate perforating guns to accomplish the formation of the perforations. For example, perforating guns have been actuated: (1) electrically; (2) through drop bar mechanisms; and (3) through pressure actuated mechanisms.
One commonly used technique for conveying the perforating guns and associated apparatus into the well is to assemble the same on a tubing string thus providing what is commonly referred to as a tubing conveyed perforating system. Such tubing conveyed perforating systems are available from the Halliburton Reservoir Services division of Halliburton Company, the assignee of the present invention.
One commonly used operating system for tubing conveyed perforating systems is a firing head which operates in response to a pressure differential. The pressure differential is created by applying increased pressure either to the tubing string or to the annulus surrounding the tubing string and conveying that increased pressure to an actuating piston contained in the firing head. Typically, such a firing head will have hydrostatic pressure balanced across the actuating piston as the tool is run into the well. When it is desired to operate the tool, increased pressure is applied to one side of the actuating piston. Thus, the low pressure reference for the actuating piston is hydrostatic pressure, and a pressure differential is created by increasing pressure on the high pressure side of the piston above hydrostatic pressure. One example of such a system is the Vann Systems differential firing head shown at page TCP-1020 of Vann Systems Engineered Well Completion Product Catalog.
Another approach of the prior art has utilized an isolated atmospheric chamber contained within a firing head as a low pressure reference zone. For example Vann Systems Pressure Actuated Firing Head shown at page TCP-1022 of Vann Systems Engineered Well Completion Product Catalog illustrates a firing head operating in response to increased tubing pressure which creates a pressure differential as compared to an atmospheric pressure chamber which is in constant communication with the low pressure side of the actuating piston.
The prior art also includes dual firing heads such as Vann Model APF-C as shown at page TCP-1028 of Vann Systems Engineered Well Completion product catalog.
There are sometimes disadvantages to using firing heads which require substantial pressure to be applied to the tubing or well casing to provide the increase in pressure which actuates the tool. In some instances, the pressures necessary to actuate the tools may be excessively high. Also, in many well perforation jobs it is desirable to perforate in an underbalanced condition, that is with a relatively low pressure present in the well when perforating occurs, and thus if high pressures are applied to actuate the perforating gun, it is necessary to be able to bleed off those high pressures very rapidly before the well is actually perforated.
On the other hand, in many situations it is undesirable to use drop bar actuated firing heads or electrically actuated firing heads.
Thus it is seen that there is a need for a pressure actuated firing system which can avoid or eliminate the application of excessively high pressures.